Zeolite minerals are framework aluminosilicates with an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing of all of the oxygen atoms. This forms a molecular structure with numerous apertures and channelways. The net negative charge of the molecular framework is balanced by the presence of alkali and alkaline earth cations. Calcium, sodium and potassium are loosely bonded to the crystal structure and are free to exchange with cations in solution. Thus, natural zeolites should have good ion exchange properties.
Cation exchange capacity (CEC) is measured by the number of channel situated cations by unit weight that may be replaced by other ions in solution. High CEC in zeolite minerals is favored by a low silica to alumina ratio. Exchange capacity is also higher in monomineralic zeolites with high sodium and low amounts of other exchangeable ions. The zeolite should be hard to provide good resistance to attrition and should have a porous and permeable network of crystals in order to provide rapid diffusion of ions in solution.
Natural zeolites with all of these ideal characteristics are uncommon.
The synthetic zeolites (manufactured analogues of natural zeolites) are molecularly engineered products formed using various compositions of clay, alumina, sodium silicate, caustic soda, etc. designed to have better ion exchange properties than natural zeolites. Synthetic zeolites are widely used in commerce for catalysts of hydrocarbons, adsorption and ion exchange.
Thus, natural zeolites in spite of favorable cost factors are not widely used for removal of metal ions from waste streams. This may be attributed to low ion exchange capacities and to the variability of chemical and physical properties in certain natural zeolites even within a given zeolite deposit. Natural zeolites usually contain several exchangeable cations species. They also usually contain trace amounts of clay, feldspar, silica, and calcite that block channelways resulting in reduced ion exchange well below theoretical capacity. These drawbacks make the natural zeolites highly inferior to synthetic zeolites.
According to the present invention natural zeolite mineral fines are formed into pellets with lignosulfonate as a binding agent and heated until the lignosulfonate is carbonized. The resulting pellets are hard, permeable and water insoluble and show improved ion exchange capabilities and rates of ion exchange as compared to untreated zeolite granules. The pellets demonstrate a high affinity for the removal of lead, cadmium and other metal and non-metal ions from waste water streams.
The process of the present invention for pelletizing zeolite fines with a carbon matrix enhances the ion exchange properties of the natural zeolites making their commercial use practical. The formation of pellets improves attrition resistance of friable zeolites and creates porous and permeable structures that improve ion diffusion from solution. The carbon matrix enhances the capacity of the pellets to scavenge metals from solution. Pelletized natural zeolites can be produced at a fraction of the cost of synthetic zeolites.
Thus, it is an object of the present invention to provide an effective natural zeolite ion exchange medium in pellet form.
It is a further object of the present invention to provide an inexpensive method of producing such effective ion exchange media from natural zeolite fines previously considered waste products.
These and other objects and advantages of the present invention will become apparent from the following detailed description of the preferred embodiment.